How Coastal Seagrass Feeds the Deep

David Pawson is a Senior Research Scientist at the National Museum of Natural History, Smithsonian Institution. He received his Bachelor of Science, Master of Science and Doctor...

Feb 11, 2013

Seagrass meadows, such as this one composed of turtle grass and manatee grass, are an important shallow water habitat.

CREDIT:

Heather Dine, Florida Keys National Marine Sanctuary

It is a well-known fact that for animals living in the deep sea, food can be scarce. The food that is around usually rains down from above as dead animals and organic particles from plankton living near the ocean’s surface. Occasionally, a bonus in the form of a good-sized dead fish, a porpoise, or even a whale will come down, the whale providing food for millions of animals for scores of years.

Marine plants, seaweeds and sea grasses, can also play an important role in providing food to the deep sea. Turtle grass (Thalassia testudinum), which grows in coastal warm waters around Florida, the Gulf of Mexico, and the Caribbean, forms extensive beds down to depths of about 30 feet (10 meters). Pieces of turtle grass are torn off by waves, drift out to sea, become waterlogged, and sink to the deep-sea floor.

Given the scarcity of food in the deep sea, it is not surprising that turtle grass is eaten by a variety of deep-sea denizens, including fast-moving sea urchins such as Hygrosoma petersii (in the order Echinothurioida). Hygrosoma often occurs in large herds, and this species is a conspicuous member of seafloor community, found at depths of 400 to more than 12,000 feet deep (130-3,700 meters). When we looked inside some of these animals, we were not surprised to see some turtle grass in their intestines, but we were truly astonished to see that their intestines contained almost nothing but turtle grass. Turtle grass is their No. 1 preferred food item!

Several scientists have noted that the turtle grass beds are disappearing, due mainly to excess nutrients—which is often caused by fertilizers and other chemicals flowing from the land to the sea. Just as the grass in an over-fertilized lawn will die, turtle grass dies in the presence of an excess of chemicals. In some places, more than 70 percent of the area of turtle grass beds have disappeared in recent years. This means that the amount of preferred food for the urchin Hygrosoma and its relatives has also decreased by 70 percent. Are these animals turning to other food sources? Are there, in fact, any other food sources?

Unfortunately, it is difficult to follow up on this problem and determine exactly how the diet of these deep-sea animals may have changed in recent years. This is because it’s very difficult to track where turtles grass goes when it floats away, and renting submersibles to research this question is very expensive. It’s possible that the urchins and other turtle grass-dependent organisms have switched their diets, perhaps to a less “tasty” food source such as the floating sargassum weed, which also sinks to the deep-sea floor.

This story demonstrates how something that happens in knee-deep water can directly affect animals living miles out to sea, and miles deep in the ocean. The deep sea is not as remote as you might think!